The rapid increase in the use of fossil fuels has accelerated the demand for alternative energy sources or clean energy sources, and research has been actively carried out into power generation and power storage using electrochemistry.
A typical example of an electrochemical device using such electrochemical energy is a secondary battery, which has been increasingly used in various fields.
Based on the shape of a battery case, secondary batteries are classified into a cylindrical battery having an electrode assembly mounted in a cylindrical metal container, a prismatic battery having an electrode assembly mounted in a prismatic metal container, and a pouch-shaped battery having an electrode assembly mounted in a pouch-shaped case made of an aluminum laminate sheet.
Based on the structure thereof, an electrode assembly, configured to have a positive electrode/separator/negative electrode structure, which constitutes a secondary battery, is classified as a jelly-roll (wound) type electrode assembly or a stacked type electrode assembly. The jelly-roll type electrode assembly is manufactured by coating a metal foil, which is used as a current collector, with an electrode active material, drying and pressing the metal foil, cutting the metal foil into the form of a band having a predetermined width and length to form a positive electrode and a negative electrode, stacking the positive electrode and the negative electrode in the state in which a separator is disposed between the positive electrode and the negative electrode, and winding the positive electrode, the separator, and the negative electrode, which are stacked, in a spiral fashion. The jelly-roll type electrode assembly is suitable for a cylindrical battery. However, it is difficult to apply the jelly-roll type electrode assembly to a prismatic battery or a pouch-shaped battery due to problems, such as separation of the electrode active material and low space utilization. On the other hand, the stacked type electrode assembly, which is configured to have a structure in which a plurality of unit positive electrodes and a plurality of unit negative electrodes are sequentially stacked, has an advantage in that it is easy to configure the stacked type electrode assembly in a prism shape.
In addition, there has been developed a stacked/folded type electrode assembly having an improved structure, which is a combination of the jelly-roll type electrode assembly and the stacked type electrode assembly. The stacked/folded type electrode assembly is configured to have a structure in which a plurality of full cells, each of which basically has a positive electrode/separator/negative electrode structure and has a predetermined unit size, are arranged, or a plurality of bi-cells, each of which basically has a positive electrode (or negative electrode)/separator/negative electrode (or positive electrode)/separator/positive electrode (or negative electrode) structure and has a predetermined unit size, are arranged, is folded using a long continuous separation film. The details of the stacked/folded type electrode assembly are disclosed in Korean Patent Application Publication No. 2001-82058, No. 2001-82059, and No. 2001-82060, which have been filed in the name of the applicant of the present patent application.
Meanwhile, in the case in which a sharp needle-shaped conductor having high electrical conductivity, such as a nail, penetrates into the electrode assembly, the positive electrode and the negative electrode of the electrode assembly are electrically connected to each other by the needle-shaped conductor, with the result that current flows through the needle-shaped conductor, the resistance of which is low. At this time, the electrodes through which the needle-shaped conductor has penetrated are deformed, and high resistance heat is generated due to conducting current in a contact resistance portion between a positive electrode active material and a negative electrode active material. In the case in which the temperature in the electrode assembly exceeds a critical temperature level due to the resistance heat, the positive electrode and the negative electrode contact each other due to shrinkage of the separator, with the result that a short circuit occurs. Such a short circuit causes a thermal runaway phenomenon. As a result, the electrode assembly and a secondary battery including the electrode assembly may catch fire or explode.
In addition, in the case in which the electrode active material or the current collector bent by the needle-shaped conductor contacts opposite electrode facing the electrode active material or the current collector, heat higher than the resistance heat is generated, whereby the thermal runaway phenomenon may be further accelerated. These problems may be more serious in a bi-cell including a plurality of electrodes and an electrode assembly including the same.
In order to solve the above problems, attempts to contain a positive temperature coefficient (PTC) material, which exhibits uniform conductivity at the general operating temperature of a battery and the resistance of which is abruptly increased to interrupt the flow of current when the temperature in the battery is increased, in the electrode have been made. In the case in which an additional layer made of the PTC material is formed on the electrode, the process of manufacturing the battery is complicated, with the result that the cost of manufacturing the battery is excessively increased. In addition, the binding force between the PTC material layer and the electrode active material layer is low, with the result that the PTC material layer and the electrode active material layer may be separated from each other.
Therefore, there is a high necessity for technology that is capable of reasonably reducing the cost of manufacturing the electrode containing the PTC material and securing the binding force between the PTC material layer and the electrode active material layer.